Lesson map · Swagger Petstore · עברית
Start from checkpoint 2. We already have a Pet[]. Now use that array as ordinary Java data.
Predict before coding: if pets[0].name changes, should the JSON response string also change? Should the server’s animal change?
Add three actions
Add these buttons immediately before the table heading, and connect them inside onCreate. Add the two new methods alongside the parser and renderer.
app > res > layout > activity_main.xml
Apply these additions and removals; keep the surrounding template code. Lines beginning with + are added and lines beginning with - are removed. Do not type the markers.
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Deserialize to Pet[]" />
+ <Button
+ android:id="@+id/showAvailable"
+ android:layout_width="match_parent"
+ android:layout_height="wrap_content"
+ android:text="Show available objects" />
+ <Button
+ android:id="@+id/showAll"
+ android:layout_width="match_parent"
+ android:layout_height="wrap_content"
+ android:text="Show all objects" />
+ <Button
+ android:id="@+id/renameFirst"
+ android:layout_width="match_parent"
+ android:layout_height="wrap_content"
+ android:text="Rename first object to Pixel" />
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
app > kotlin+java > com.example.topic > MainActivity
Apply these additions and removals; keep the surrounding template code. Lines beginning with + are added and lines beginning with - are removed. Do not type the markers.
import com.google.gson.Gson;
import com.google.gson.JsonParseException;
+import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
import java.util.Map;
binding.getPets.setOnClickListener(v -> getPets());
binding.loadDemo.setOnClickListener(v -> loadDemo());
binding.parsePets.setOnClickListener(v -> parsePets());
+ binding.showAvailable.setOnClickListener(v -> showAvailable());
+ binding.showAll.setOnClickListener(v -> renderPets(pets));
+ binding.renameFirst.setOnClickListener(v -> renameFirst());
}
/** Fetches JSON as text; no Java Pet objects are created here. */
}
}
+ /** Selects existing objects by their status; no JSON processing is needed. */
+ private void showAvailable() {
+ ArrayList<Pet> available = new ArrayList<>();
+ for (Pet pet : pets) {
+ if ("available".equals(pet.status)) available.add(pet);
+ }
+ renderPets(available.toArray(new Pet[0]));
+ }
+
+ /** Mutates an object in memory while leaving both JSON text areas untouched. */
+ private void renameFirst() {
+ if (pets.length == 0) {
+ binding.objectStatus.setText("Deserialize some animals first");
+ return;
+ }
+ pets[0].name = "Pixel";
+ renderPets(pets);
+ }
+
/** Displays object fields; this method never reads or parses JSON. */
private void renderPets(Pet[] animals) {
binding.petTable.removeAllViews();
Follow object identity
showAvailable reads each object’s status and collects matching objects. It neither downloads nor deserializes anything. The new ArrayList contains references to existing animals. toArray(new Pet[0]) gives the renderer an array; Show all renders the original array again.
renameFirst changes the first object’s field, then rebuilds the table. It does not change either text area and sends no request. Rendering a filter does not change the stored pets array, so Rename first always means the first animal in that original array, not necessarily the first filtered row. It returns to the full table to make the result clear.
Build and prove the difference
- Load the demo and deserialize: three animals.
- Show available: Milo and Nala. Show all: all three return.
- Rename first: the table now says Pixel. The editable JSON still says Milo. The last HTTP body is also unchanged.
- Deserialize the unchanged sample again: Milo returns, because you reconstructed new objects from the old text.
- Change Luna’s status to available in Notepad++, paste the array, deserialize, and filter. Predict the count before pressing the button.
Why this demonstrates the power of deserialization
Once text becomes objects, the program can use familiar fields, conditions, loops, arrays and methods. You do not write a string-splitting parser each time you want to filter, rename, count or display animals. The table is a view of those objects, not the data itself. Serialization will let us make a new text representation of the changed objects.